Chemical plating method, electrolytic cell and automotive oxygen sensor using it

Abstract

An automotive lambda oxygen sensor is formed by electroless plating of a thin, catalytically active, conductive electrode uniformly on the outer surface of a zirconia thimble. The process includes forming a pristine zirconia solid electrolyte thimble and drilling out a cylindrical cavity in it. A porous outer surface suitable for producing crystallization sites is formed by dipping the unfired thimble in a zirconia slurry containing spray-dried microspheres and firing the coated thimble to densify the thimble and the microspheres and to produce cavities on the surface of the thimble. An inner platinum electrode is formed by conventional conductive ink painting on the axial cavity of the sensor, and the sensor is again fired. The surface is activated by immersion in an acetone chloroplatinic acid bath to form multiple crystallization points, heat treated, then plated in an electroless platinum bath to a desired thickness. After plating, the sensor is heat treated and a conventional spinel glaze coat is flame sprayed over the sensor. The process produces sensors which consistently provide rapid response times and stable operation.

We claim:  
     
       1. A method of forming a solid electrolyte cell comprising forming a solid electrolyte body as a thimble with an outer surface and an inner surface, forming a porous layer on the outer surface of the body, activating the porous layer on the outer surface of the body to form a plurality of growth points for a conductive layer on the outer surface, growing a first electrode by electroless plating of a conductive layer on the activated porous layer on the outer surface of the body, and forming a second electrode on the inner surface of the body, the method further comprising drilling axial cavity in the thimble, and thereafter firing the body to densify it. 
     
     
       2. The method of claim  1  wherein the step of forming a solid electrolyte body comprises forming a body which is impervious to air. 
     
     
       3. The method of claim  1  wherein activating the porous layer on the outer surface comprises wicking a metal salt carried by a liquid into the porous layer. 
     
     
       4. The method of claim  1  wherein growing a first electrode comprises immersion of the porous layer on the outer surface in an unstable solution of a salt of a metal. 
     
     
       5. The method of claim  1  wherein the body is formed by uniaxially compressing a zirconia powder into a thimble having a tapered bore, and then drilling out the tapered bore to form a substantially cylindrical cavity. 
     
     
       6. The method of claim  1  further comprising inserting into the thimble an elongate electrical terminal extending from outside the cell into the interior of the thimble, the terminal including a pair of arms, at least one of the arms engaging the second electrode. 
     
     
       7. The method of claim  6  further comprising inserting into the thimble an elongate electrical heater extending into the interior of the thimble, the terminal arms embracing the heater and positioning the heater in the thimble. 
     
     
       8. The method of claim  3  wherein the liquid is an organic solvent which wets the ceramic. 
     
     
       9. The method of claim  8  wherein the organic solvent is acetone. 
     
     
       10. The method of claim  3  including a step, after wicking the solution into the pores at the surface of the substrate, of heating the substrate to drive off the liquid and reduce the salt to a 0.01 to 0.5 micron layer of a metal with numerous unplated areas.